U.S. President Barack Obama said on December 14 that Islamic State (IS) militants have lost large swaths of territory that they once controlled in Iraq and Syria, but more progress needs to be made faster against the extremist group.

Speaking to reporters at the Pentagon, Obama said he has tasked Defense Secretary Ash Carter with going to the Middle East to secure more military contributions from other countries in the U.S.-led coalition that is fighting against IS.

Working to easy public uneasiness ahead of the holidays, Obama held a meeting of his National Security Council at the Pentagon, a rare move, as part of a push aimed at explaining his strategy to stop IS militants abroad and the group’s sympathizers within the United States.

He told reporters after the Pentagon meeting that the U.S. strategy includes hunting down militant leaders, training security forces in other countries, and stopping the group's financing and propaganda.

Obama revealed that a group of U.S. special operations commandos has started working with local fighters in Syria to "tighten the squeeze" on Raqqa, the nominal capital of IS militants, and said intensified bombing of Syria’s oil infrastructure is targeting IS revenue streams.

"As we squeeze its heart, we'll make it harder for ISIL to pump its terror and propaganda to the rest of the world," Obama said, using an alternate acronym for the extremist group.

Obama’s administration announced in late October that he had approved deploying up to 50 special operations troops to Syria on the first open-ended mission by U.S. ground forces in the country.

U.S. officials had previously refused to say whether the U.S. commandos had started their mission in Syria.

Obama pledged to kill IS leaders and regain territory from the militant group the Middle East.

"ISIL leaders cannot hide, and our next message to them is simple: You are next," he said.

He added that since the summer, IS militants have "not had a single successful major offensive operation on the ground in either Syria or Iraq."

"We are hitting ISIL harder than ever," he said.

He said, however, that "we recognize that progress needs to keep coming faster."